Compensation of reactive power flows in electric power networks conventionally occurs, inter alia, by connection of reactive impedance elements, whereby is meant in this context inductors and capacitors, in shunt connection to the power network. By connection of a thyristor switch, essentially comprising two thyristor valves in antiparallel connection, in series with an inductor, the current through the inductor may be controlled and hence also the exchange of reactive power by the device with the power network (Thyristor Controlled Reactor—TCR). In a similar manner, capacitors may be respectively connected to and disconnected from the power network by means of electric switching devices, for example thyristor switches, whereby the reactive power supplied to the power network may be controlled in steps (Thyristor Switched Capacitor—TSC). Fixed capacitors also often occur in combination with TCRs and combinations of TSCs and TCRs, which makes possible a continuous control of the reactive power exchange with the power network.
Capacitors connected in shunt connection are used primarily in industrial networks to compensate for the reactive power consumption in, for example, large asynchronous motors. Other typical applications, where often a combination of fixed capacitors and TCRs is advantageous, is in connection with loads with a greatly varying reactive power consumption, such as, for example, in arc furnaces. In certain cases, it may be suitable to connect the compensation equipment to the industrial network via a transformer.
With the arrival of Voltage Source Converters (VSCs), equipped with series-connected transistors (IGBT), it has been possible to use this kind of converters for relatively high voltages. Control by means of pulse-width modulation (PWM) allows a rapid control of the voltage generated by the converters. Converters of this kind thus constitute a device that enables a rapid control of a generated ac voltage, both with respect to amplitude and phase position.
With the converter connected to a power network with a certain fundamental frequency, nominally usually 50 or 60 Hz, the voltage generated by the converter is brought to comprise a component of the fundamental frequency, in the following referred to as the fundamental voltage, but in addition thereto, because of the pulse-width modulation, also components of other frequencies.
However, in the following, only the fundamental voltage is taken into consideration.
It is known to connect such a voltage source converter to an industrial network to achieve a rapidly controllable reactive power exchange with the network—see, for example, ABB Review 6/98 pp 21–30: SVC Light—A powerful tool for power quality improvement (Bo Bijlenga, Rolf Grü nbaum, Thomas Johansson).
The connection of the converter is made in these known applications via phase inductors, which are normally dimensioned such that, at the rated current of the converter, they take up about 10–30% of the network's nominal voltage of fundamental frequency. The converter is brought to generate a voltage, the fundamental component of which, both with regard to frequency and phase position, essentially coincides with the voltage of the network (to cover active losses in converters and phase inductors, the phase position must deviate somewhat from the phase position for the voltage of the network; this is disregarded in this reasoning on principles), and by varying the amplitude of the generated voltage, the converter may be brought to consume reactive power, if its voltage has a lower amplitude than that of the network, and to generate reactive power, respectively, if its voltage has a higher amplitude than that of the network. Since, in industrial networks, the task is normally to generate reactive power, the voltage source converter is normally supplemented by a capacitor, which may possibly be connectible in steps.
Thus, according to the above-mentioned technique, the converter must be dimensioned for a voltage equal to the nominal voltage of the network plus a control range for generation of reactive power.
However, it often proves that such a configuration leads to the converter not being capable of being utilized in full with respect to its capacity to carry current, and this also means that it becomes overdimensioned with respect to current. To achieve a better utilization of the converter, it may then be connected via a transformer, which changes the ratio of current to voltage in the converter and hence enables a better utilization. This, however, entails an additional component in the installation.
Inductors coupled in shunt are primarily used in high-voltage transmission networks with overhead lines but also in transmission networks with cables, in the latter case also at lower voltages.
The purpose of this is above all to suppress overvoltages that may arise in connection with switching operations in the transmission network and to counteract voltage rises along a transmission line at low load. On the other hand, this means that the inductor, at increasing load, contributes to a non-desired voltage drop along the line.
It is known to use an inductor, connectible by means of a switch, with a fixed inductor to counteract voltage variations along the line at varying load. However, this control of the reactive power exchange with the transmission network is discontinuous and, in comparison with the above-mentioned overvoltages, relatively slow. In this case, the task of the fixed inductor is to reduce overvoltages in connection with sudden voltage rises caused, for example, by switching operations in the power network. The fixed inductor may be equipped with a thyristor switch of the kind mentioned above in connection with TCRs. Such a solution, however, entails a limited control speed, caused by the mode of operation of the thyristor switch.